This invention relates to an optical information recording medium such as a compact disc (namely, CD), a digital video disc (namely, DVD), a recordable compact disc (namely, CD-R) or a recordable digital video disc (namely, DVD-R).
An optical information recording medium on which information can be only once recorded by means of a laser beam (i.e., an optical disc of write-once type) is known as a recordable compact disc (CD-R), and at present is widely used in practice. The optical disc of CD-R type generally has a multi-layered structure typically comprising a disc-shaped transparent substrate (support), a recording layer comprising an laser light-sensitive organic dye (often referred to as xe2x80x9crecording dye layerxe2x80x9d), a light-reflecting layer comprising a metal (hereinafter, often referred to as xe2x80x9cmetallic light-reflecting layerxe2x80x9d, or simply xe2x80x9creflecting layerxe2x80x9d), and optionally a protective layer made of resin overlaid in order.
A demand for large recording capacity has recently increased, but an optical disc of CD-R type does not have satisfactorily large information-recording capacity. Therefore, an optical disc having satisfactorily large information-recording capacity has been studied and proposed. For example, a recordable DVD (i.e., recordable digital video disc, DVD-R), in which information can be recorded on and read out from by means of a laser beam having a shorter wavelength than that for CD-R, has been developed. The optical disc of DVD-R type is formed by laminating two composites having a layered structure. Each of the layered composites comprises a transparent disc substrate provided with a guide groove (i.e., pre-groove) for tracking of the laser beam, a recording layer comprising an organic dye, a light-reflecting layer, and a protective layer arranged in order. The layered composites are combined with an adhesive so that the recording layers would be placed inside. Otherwise, the optical disc of DVD-R type is formed by combining a protective disc plate and the layered composites comprising a substrate, a recording layer, a light-reflecting layer, and a protective layer arranged in order.
For writing (i.e., recording) and reading (i.e., a reproducing) the information, a CD-R is irradiated with a laser beam having a wavelength in the range of 770 to 790 nm, typically approx. 780 nm, while a DVD-R is irradiated with a laser beam of visible wavelength region (generally having a wavelength of 600 to 700 nm, typically approx. 650 nm). By the irradiation of the laser beam, the irradiated area of the recording dye layer is locally heated to change its physical or chemical characteristics. Thus, pits are formed in the irradiated area of the recording layer. Since the optical characteristics of the formed pits are different from those of the surrounding area having been not irradiated, the digital information is recorded. The recorded information can be read by a reproducing procedure generally comprising the steps of irradiating the recording layer with a relatively weak laser beam having the same wavelength as that employed in the recording procedure, and detecting the light-reflection difference between the pits and their surrounding areas.
The recording dye layer of DVD-R or CD-R is generally formed by dissolving a laser light-sensitive dye in a solvent to prepare a dye solution and coating the dye solution on a transparent substrate disc having a pre-groove on its surface by spin-coat procedure. In the spin-coat procedure, the dye solution is continuously dropped on the surface of the substrate under rotation. The dropped dye solution is centrifugally spread out, and an excessive portion of the dye solution is splashed off from the edge of the substrate. The dye solution remaining on the substrate is dried by evaporation of the solvent to form a thin recording dye layer on the substrate.
The solvent for the dye solution naturally should dissolve therein the laser light-sensitive dye at a relatively high solubility level, but essentially should not dissolve the material of the substrate. Most of the known solvents in which the dye is well soluble dissolve the surface of the substrate to damage the pre-groove. Most of the known solvents which are essentially inert to the substrate cannot dissolve the dye at a satisfactory level.
U.S. Pat. No. 4,832,992 discloses that a fluorine-coating compound is favorably employed as the solvent for the preparation of the dye solution. The fluorine-containing compound has excellent properties as a solvent for the preparation of a dye solution. For example, most laser light-sensitive dyes are well soluble in the fluorine-containing compounds while a substrate made of polycarbonate is not damaged by the fluorine-containing compounds. The U.S. Patent further discloses a representative fluorinated alcohol, namely, 2,2,3,3-tetrafluoro-1-propanol.
Thus, the fluorinated alcohol is a favorable solvent for the preparation of a dye solution to be coated on a substrate disc made of polycarbonate. However, our detailed study has revealed that it takes a relatively long time of period for evaporating the fluorinated alcohol from the dye solution coated on the substrate disc to produce a uniform thin recording dye layer. The requirement of such long time of period for drying the coated dye solution is disadvantageous for mass production of optical recording discs.
Accordingly, the present inventors have made studies to find a group of new solvents which can dissolve the laser light-sensitive dye at a relatively high level, which is essentially inert to the substrate material, particularly, polycarbonate, and which evaporates rapidly at an ambient temperature.
In the course of the studies, the inventors have tried to employ an organic solvent comprising a fluorinated cyclic alkane or alkene as a solvent for preparing a dye solution for the manufacture of a recording dye layer of an optical information recording medium.
The fluorinated hydrocarbon such as fluorinated cyclic alkane or alkene is described for the use as a detergent in Japanese Patent Provisional Publication Nos. 10-316596, 10-316597 and 10-316598. These publications further describe that a 4- to 6-membered fluorinated cyclic alkane or alkene, particularly, 1,1,2,2,3,3,4-hepta-fluorocyclopentane is preferred.
Accordingly, it is the object of the present invention to provide a laser light-sensitive dye solution which can dissolve the dye at a relatively high level, which is essentially inert to the substrate material, particularly, polycarbonate, and which evaporates rapidly at an ambient temperature.
It is another object of the invention to provide a process for the production of an optical information recording medium which is enable to produce the recording medium for a shortened drying period.
The present invention resides in a process for the preparation of an optical information recording disc comprising the steps of coating a dye solution on a transparent, substrate disc and drying the coated dye solution, in which the dye solution is a solution of a laser-light sensitive organic dye dissolved in an organic solvent comprising a fluorinated cyclic alkane or alkene.
The invention further resides in an optical information recording disc comprising a transparent substrate disc and a recording dye layer provided on the substrate disc, in which the recording dye layer is produced by the above-mentioned process.
The invention furthermore resides in a dye solution which comprises a laser-light sensitive organic dye dissolved in an organic solvent comprising a fluorinated cyclic alkane or alkene.
In the dye solution of the invention, the fluorinated cyclic alkane or alkene preferably has fluorine atoms of a number of 0.5 to 2.0 times based on the number of carbon atoms constituting its skeleton. Most preferred is 1,1,2,2,3,3,4-heptafluorocyclopentane, which is illustrated by the following formula (I). 
The dye solution of the present invention is preferably prepared by the steps of mixing the organic dye and the solvent and keeping the resulting mixture at a temperature of higher than 45xc2x0 C. but lower than a boiling temperature of the solvent by at least 10xc2x0 C. for a period of 5 minutes to 2 hours.
The solvent for the preparation of the dye solution of the invention may comprise the fluorinated cyclic alkane or alkene and one or more organic solvents or liquids. For instance, the solvent may comprise the fluorinated cyclic alkane or alkene and an organic liquid which has a boiling point higher than that of the fluorinated cyclic alkane or alkene, which is essentially inert to the substrate, and in which the dye is soluble, in a volume ratio of 99:1 to 51:49. The organic liquid preferably has a boiling point of lower than 250xc2x0 C. but higher than that of the fluorinated cyclic alkane or alkene by at least 10xc2x0 C. Examples of the organic liquids include an aliphatic ketone, an aliphatic hydrocarbon, an alicyclic hydrocarbon, a carboxylic acid ester, an aliphatic ether, and an alcohol. Preferred organic liquids are a fluorinated alcohols having the formulas of CF3CH2OH, HCF2CF2CH2OH, H(CF2CF2)2CH2OH, and H(CF2CF2)3CH2OH. A keto-alcohol (e.g., diacetone alcohol) is also preferred.
The organic solvent may comprise the fluorinated cyclic alkane or alkene and an organic liquid which has a solubility for the dye higher than that of the fluorinated cyclic alkane or alkene and which is active to the substrate, in a volume ratio of 99.9:1 to 80:20. Examples of the organic liquids include a halogenated aliphatic hydrocarbon, an aromatic hydrocarbon, a ketone, a sulfoxide, an amide, a carboxylic acid ester, an ether, and a nitrile. Preferred organic liquid is a halogenated aliphatic hydrocarbon such as dichloromethane, dichloroethane, or tetrachloroethane. Acetone is also preferred.
The solvent may comprise the fluorinated cyclic alkane or alkene and a fluorinated alcohol in a volume ratio of 50:50 to 1:99. Preferred fluorinated alcohol is 2,2,3,3-tetrafluoro-1-propanol.